In order to take into account longitudinal streaks or other profile variations, the sheet property of interest needs to be determined at a plurality of locations spaced across the width of the sheet. To this end, the sensor or sensors are commonly provided with a traversing mount that supports and guides the sensors for movement transverse to the nominal sheet path. This movement is generally along a nominal sensor path or paths established by the design of some kind of carriage and the location of some kind of track member or members on which the carriage rides.
The sheet may be constrained to move over the surface of a roll or some other surface that establishes a reference position in the region where the sheet is to be inspected. More commonly the sheet is unsupported in the inspection region and is subject to deviations from the nominal sheet path. Deviations of the sheet from a fixed plane of nominal sheet movement at the sensor location are referred to as "flutter."
The nominal sensor path is usually spaced from, and orthogonally parallel to, the nominal sheet path. However, the actual sensor path is subject to deviations from the nominal sensor path. These deviations are caused, for example, by deflections of the track members due to the shifting weight of the movable sensor and carriage, localized or general temperature variations, operation of the carriage drive mechanism, vibrations of processing machinery and the like.
The response of almost all basic sheet sensors are influenced by changes in the relative positions of the sensor and the sheet. To minimize the effect of these changes on the response of the sensor, the path of the sensor is sometimes effectively constrained to the path of the sheet, as described, for example, in U.S. Pat. No. 3,369,408, or the path of the sheet is sometimes effectively constrained to the path of the sensor means, as described, for example in U.S. Pat. No. 3,818,327.
Some of the most common types of sheet sensors utilize one or more forms of radiant energy, such as visible light, infrared, microwave, beta, gamma or X radiation. Basically the response of these sensors is influenced by changes in the distance between sensor elements such as the radiation source and the radiation detector, as well as between the source and the sheet and between the detector and the sheet.
Traversing mounts for such sensors commonly include elaborate provisions for minimizing deviations of the sensor parts from their nominal paths. As described in U.S. Pat. No. 3,668,397, for example, the sensor carriages may ride on steel tubes that are stretched under tension like a bowstring, with auxiliary supports that are occasionally or periodically realigned automatically to restore straightness. Where the contour of the nominal sheet path is normally curved across the sheet width, a matching curvature may be imparted to the nominal sensor path, as described, for example, in U.S. Pat. No. 3,191,034.
Changes in the distance between elements of a sensor means, e.g., a radiation source and a detector, or between a sensor means and a primary reference surface, e.g., a roll over which the sheet passes during measurement, are sometimes deemed unavoidable, or it may be considered uneconomical to provide the expensive structures and methods required to maintain the desired constant spatial relationships. What is sometimes done in this case is to produce a virtual or actual recording of the changes in the response of the sensor when the sheet is absent from its normal sheet path and while the sensor is moved between the limits of its normal sensor path. The recorded sensor output changes (as a function of the sensor position) are subsequently applied to the sheet property measurements obtained when the traveling sheet occupies its normal path, either as a computed correction to the penultimate measurement response or as an applied variation to a component of the measuring instrument, as described, for example, in U.S. Pat. No. 3,306,103.
These prior art "air profile" compensation techniques, however, do not take into account certain changes in conditions that prevail when the traveling sheet is present. The changed conditions may include, for example, thermal effects on the traversing mount structures due to heat-shielding of the structures by the sheet. Thermal effects can also result from the heat-exchange relationships of the structures with the traveling sheet and the boundary layer of air that may be entrained with it. The traversing mount structures can also be affected by vibrations and deformation of adjacent structures that may occur when the sheet processing machinery resumes operation.
Accordingly, many radiation source and detector arrangements have built-in flutter and deflection compensating geometric structures that allow an appreciable degree of flutter and deflection to take place with a minimal effect on the sensor response. The compensating geometric structures in combination with extremely stable traversing mounts have provided excellent results from sensor instruments utilizing radiation absorption and scattering phenomena. On the other hand, the compensating geometric structures have imposed certain constraints on the capabilities of these instruments, and much more versatile systems can be made available if these constraints can be relaxed.
Moreover, it is frequently desired to measure the thickness of sheets, either as a caliper measurement per se or as a caliper measurement in combination with the simultaneous measurement of other sheet properties. Thickness can be measured with an optical sensor that uses a well known optical triangulation method in order to respond basically to the distance from the sensor to one surface of the sheet. It is required, however, to know exactly the relative positions of the sensor and the opposite surface of the sheet. If the opposite surface of the sheet can be kept in constant contact with a fixed reference surface, and if the distance from the reference surface to the optical sensor can be kept constant, the distance measurement can be interpreted as thickness, as disclosed for example in U.S. Pat. No. 3,858,983. Where the sheet is unsupported in the measuring region, two basically identical optical distance sensors on opposite sides of the sheet can be used as described in U.S. Pat. No. 3,565,531 to derive a thickness measurement from two distance measurements.
The apparatus of the latter patent utilizes fixed structures on the two sides of the sheet, and measures thickness at only a single point across the width of the sheet. If it is desired to measure at several points across the sheet width, a multiplicity of distance sensors may be used, and several sensors may share the use of a single laser beam source of radiation, as disclosed in U.S. Pat. No. 3,671,726. However, for certain important measurements and analyses, such as resolving the variation components of the measured sheet property in both the machine direction and the cross direction, for example, as described in U.S. Pat. Nos. 3,552,203 and 3,612,839, it appears that the number of such sensors required would become both impractical and prohibitively expensive. On the other hand, proposals as in U.S. Pat. No. 3,858,983 supra for the use of a single optical distance or thickness sensor arrangement on a traversing gauge mount have not taken into account the ubiquitous and quite serious problems caused by the position instability of the mechanical sensor support and guidance systems.